This invention relates to a four-wheel drive control system including a rotation transmission device mounted on the power train of a four-wheel drive vehicle for selective transmission and non-transmission of driving force, and capable of maintaining the function of the vehicle immediately after it is started in a cold environment.
If a 4-wheel drive vehicle turns a tight corner with all the wheels drivingly coupled to the engine, the vehicle will undergo what is known as the "tight-corner braking" effect. To prevent this, the present applicant proposed a rotation transmission device comprising a roller type two-way clutch and a current control means in Japanese patent applications 8-172598 and 9-28001.
This rotation transmission device A, shown in FIGS. 5 and 6, is mounted in a transfer 5 of an FR-based 4WD having hub clutches 2 mounted between the front wheel axle and front wheels 1. The output of the transmission 4 coupled to the engine 3 is always transmitted to the rear propeller shaft 8 leading to the rear wheels 7 through an input shaft 6 of the rotation transmission device A. The device A comprises the input shaft 6, a chain sprocket 9 coaxially and relatively rotatably mounted on the input shaft 6, a synchronizer 22 for synchronizing the input shaft 6 and the sprocket 9, a two-way clutch 10 with engaging elements or rollers for coupling and uncoupling the sprocket 9 and the input shaft 6, and a current control means or electromagnetic clutch 11 for locking and unlocking the two-way clutch 10. This 4WD has a 4WD control mode in addition to the drive modes available in a typical conventional part-time 4WD (i.e. 2WD, 4WD-Hi and 4WD-lo).
FIGS. 7A and 7B show the two-way clutch 10 and the magnetic clutch 11 of the rotation transmission device A. The two-way clutch 10 includes an inner member 12 and an outer ring 13 coaxially and rotatably mounted around the inner member 12 through bearings. One of the inner member 12 and the outer ring 13 is formed with a plurality of cam faces 14, and the other with a cylindrical face 15 opposite the cam faces 14 to define wedge spaces between the faces 14 and 15. A retainer 16 is received in the wedge spaces. The retainer 16 is formed with a plurality of pockets in which are received rollers 17 as engaging elements. A switch spring 18 engages at one end thereof the retainer 16 and at the other end the inner member 12 or the outer ring 13 formed with the cam faces 14 to bias the retainer 16 toward a neutral position where the rollers 17 do not engage the cylindrical face 15 and the cam faces 14 simultaneously.
The electromagnetic clutch 11 as a current control means for locking and unlocking the two-way clutch 10 comprises a friction flange 19 fixed to the outer ring 13 or the inner member 12, an armature 20 axially slidably but relatively nonrotatably coupled to the retainer 16 and provided in juxtaposition with the friction flange 19 with a gap therebetween, and an electromagnetic coil 21 for magnetically pressing the friction flange 19 and the armature 20 against each other. By selectively activating and deactivating the coil 21, the rollers 17 are engaged and disengaged.
A 4WD performs best in snowy, cold environments. Immediately after it is started in a cold condition, oil in the transfer is also cold and thus high in viscosity.
The 4WD shown has a mode changeover switch 23. When it is traveling with the switch 23 in the 2WD position, the hub clutches 2 and the two-way clutch 10 are disengaged, so that the front power train stops.
Immediately after the 4WD is started in a cold environment, oil in the transfer is cold and high in viscosity. Thus, even during the 2WD mode, in which the two-way clutch 10 is supposed to be kept unlocked with the inner member 12 rotating relative to the stationary outer ring 13, the rollers 17 may get locked by the dragging resistance of the oil in the transfer.
When the rollers 17 lock, the outer ring 13 begins to rotate together with the inner member 12. In this state, no large dragging force acts on the rollers 17, so that the rollers return to the neutral position, allowing the outer ring 13 to slow down. This causes the rollers 17 to get locked again. While the oil temperature is low, this cycle is repeated, producing vibrations.
An object of this invention is to provide a control system for a 4WD vehicle having a rotation transmission device which can prevent the two-way clutch of the rotation transmission device from vibrating by repeatedly locking and unlocking when the vehicle is started in a cold environment.